1. Field of the Invention
This invention relates to a contact material for a vacuum valve and a method of manufacturing the same.
2. Description of the Related Art
The most important properties which a contact material for vacuum valve is required to have are the three basic requirements of anti-welding property, voltage withstanding capability and current interrupting property. Further important requirements are to show low and stable rise in temperature and low and stable contact resistance. However, it is not possible to satisfy all these requirements by a single metal, as some of them are contradictory. Consequently, many of the contact materials that have been developed for practical use consist of combinations of two or more elements so as to complement their mutual deficiencies in performance, and to match specific applications such as large-current use or high voltage-withstanding use. Contact materials have been developed possessing excellent properties in their own way. However, performance requirements have become increasingly severe and the present situation is that these materials are unsatisfactory in some respects.
There has been a marked tendency in recent years to expand the range of circuits to which these materials are applied to reactor circuits and capacitor circuits etc., and development and improvement of the contact materials corresponding to these application has become an urgent task. In particular, regarding capacitor circuits, due to the application of twice the voltage of an ordinary circuit, problems have arisen in respect of the withstand voltage characteristic of the contacts, in particular of suppressing occurrence of restriking. In order to cope with this, conventionally, Cu--Cr contact material has been employed, which has excellent current interrupting property and comparatively good withstand voltage characteristics.
However, such Cu--Cr contact material can cope to some extent in the high withstand voltage field. But in more severe high withstand voltage regions and in circuits that are subject to inrush current, there is a problem of occurrence of restriking. One of the reasons why Cu--Cr contact material does not necessarily exhibit sufficient performance in the high withstand voltage region is considered to be as follows. Opening and closing of the contacts results in the formation of Cu--Cr finely dispersed layer at the contact surface, which is of mechanically higher strength than the contact material. It is believed that micro-welding locally produced by the inrush current causes the exfoliation from the contact material portion, with the formation of severe surface irregularity, causing field concentration and clump. Consequently, it is believed that the probability of occurrence of restriking should be able to be reduced by increasing the strength of the contact material.
Infiltrated Cu--Cr contact obtained by infiltrating Cu into a Cr skeleton manufactured by sintering Cr powder show a lower rate of occurrence of restriking than solid-phase sintered Cu--Cr contacts manufactured by mixing and sintering Cr powder and Cu powder. Furthermore, Cu--Cr contacts made by arc melting of a consumable electrode manufactured of Cu--Cr show even lower rate of occurrence of restriking.
However, in the Cu--Cr contacts manufactured by the consumable arc melting method, local non-uniformity in the contact micro structure is formed by the occurrence of two-phase separation of a Cu-rich liquid phase and Cr-rich liquid phase that are produced during solidification and cooling steps of the consumable arc melting method. Since this Cr-rich portion is brittle in terms of material, cracking and breaking away occur during opening and closing of the contacts, causing restriking to occur.
Hereinafter another problem of the conventional contact material will be described. The present situation is that contact materials for a vacuum valve which are able to fully satisfy increasingly severe requirements in respect of high withstand voltage property and large current interrupting capability have not yet been developed.
In recent years therefore some use has been made of contact materials combining arc-proof constituents of excellent withstand voltage performance and arc-proof constituents having excellent current interrupting performance. For example, Japanese Patent Disclosures (kokai) No. Sho. 59-81816 and No. Sho. 59-91617 disclose contact materials having prescribed contents of Ta and Nb in a Cu--Cr contact material, which have excellent current interruption performance and also improved voltage withstanding characteristics.
However, regarding contact materials for a vacuum valve as described above, with contact materials manufactured by a solid-phase sintering process, in which the conductive constituent and other arc-proof constituents are simply mixed and sintered, it can hardly be said that fully satisfactory contact materials (i.e. contact materials wherein both these characteristics are improved and stabilized) have been obtained.
Means for improving the withstand voltage characteristic and current interruption performance, in particular, a method of manufacture whereby the withstand voltage characteristic is improved are disclosed in, for example, Japanese Patent Disclosure (Kokai) No. Sho. 63-158022. However, it cannot necessarily be said that this can satisfy the requirements.